The Ammonium Transporter SpAMT1;2 Contributes to Nitrogen Utilisation and Cadmium Accumulation in the Hyperaccumulator Sedum Plumbizincicola

Sedum plumbizincicola (Sp) is a cadmium (Cd) hyperaccumulator found specifically in abandoned ancient mines where N is regularly deficient while Cd presents in excess. How Sp got adapted to this unique habitat remains unknown. Here, we reported relative abundant presence of NH₄ ⁺ in mine areas for Sp, and the isolation and functional characterisation of a putative NH₄ ⁺ transporter gene AMT1;2, which is highly expressed in Sp roots and encodes a pH-dependent dual affinity ammonium uptake transporter. Compared to SaAMT1;2, the homologous gene in the nonhyperaccumulating control Sedum alfredii (Sa), SpAMT1;2 expression is much higher and not inhibited by Cd. Only eight amino acid sequence polymorphisms were observed between SpAMT1;2 and SaAMT1;2, and the in-vitro NH₄ ⁺ uptake activity and subcellular localisation are identical between them with or without Cd stress. Moreover, in contrast in Sa, NH₄ ⁺ uptake in Sp is not inhibited by Cd, and NH₄ ⁺ at ambient level promotes Cd accumulation. These data suggest that SpAMT1;2 is likely an essential gene contributing to nitrogen nutrition and the interaction between NH₄ ⁺and Cd uptake in Sp, which might represent a novel N utilisation pathway evolved in mines for the hyperaccumulator Sp.